Fix findbar slowness

[bsclifton]

FIxes #10271

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

• Find on page should work as before
• Loading a large page and finding (while it's loading) shouldn't cause lag (eg: typing shows up immediately)

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[diracdeltas]

i think this needs a rebase

[codecov-io]

Codecov Report

Merging #10283 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10283      +/-   ##
==========================================
- Coverage   52.95%   52.93%   -0.02%     
==========================================
  Files         228      228              
  Lines       20308    20308              
  Branches     3253     3253              
==========================================
- Hits        10754    10751       -3     
- Misses       9554     9557       +3

Flag	Coverage Δ
#unittest	52.93% <ø> (-0.02%)	⬇️

Impacted Files	Coverage Δ
js/stores/appStoreRenderer.js	91.17% <0%> (-8.83%)	⬇️